1. Field of the Invention
This invention relates to the control of electronically switched, brushless machines, such as switched reluctance machines, permanent magnet machines and hybrids thereof.
2. Description of Related Art
The typical switched reluctance machine, for example, comprises a rotor, defining rotor poles, a stator defining stator poles, and a set of windings arranged in relation to the stator poles to define one or more phases. In a reluctance machine, energization of one or more phase windings sets up a magnetic flux circuit including the associated stator poles, urging the rotor into a position of minimum reluctance. Timing the sequential energization of the windings according to rotor position induces rotor movement. Switched reluctance machines are well known. More detail is provided in the paper xe2x80x98The Characteristics, Design and Applications of Switched Reluctance Motors and Drivesxe2x80x99 by Stephenson and Blake, presented at the PCIM ""93 Conference and Exhibition at Nurnberg, Germany, Jun. 21-24, 1993 which is incorporated herein by reference. As is well known in the art, these machines can be operated as motors or generators simply by altering the timing of the application of the excitation to the phase windings.
As explained in the above paper, the method of torque production in a switched reluctance machine is quite different from that in conventional machines, e.g. induction or synchronous machines, which are operated by rotating waves of magneto-motive force (mmf) and in which the torque is produced by the interaction of a magnetic field with a current flowing in a conductor. Such machines are known as xe2x80x98electromagneticxe2x80x99 machines and encompass, e.g., so-called brushless DC machines in which the current is in stator coils and the field is produced by permanent magnets on the rotor. By contrast, switched reluctance machines are purely xe2x80x98magneticxe2x80x99 machines, where the torque is produced solely by the magnetic field as the reluctance of the magnetic circuit changes. These machines require the use of permanent magnets. The rotor and the stator are made of unmagnetized, but magnetizable metal, such as electrical sheet steel which is a typical xe2x80x9csoftxe2x80x9d magnetic material. It follows that the methods of controlling the two types of machine are quite different, since the control is related to the method of torque production. In general, the control methods used for conventional sinusoidally fed conventional machines are quite inappropriate for switched reluctance machines.
FIG. 1 shows a typical switched reluctance machine in cross section. In this example, the stator 10 has six stator poles 12, and the rotor 14 has four rotor poles 16. Each stator pole carries a coil 18. The coils on diametrically opposite poles are connected in series to provide three phase windings. Only one phase winding is shown, for clarity. The control of the switched reluctance machine can be achieved in a variety of ways. The machine could be controlled in an open-loop fashion, i.e. as commonly used for stepping motors. In this regime, the phase windings in the machine are sent pulses in turn and it is assumed that the rotor lines up with each pair of stator poles in turn, i.e. the position of minimum reluctance for that phase which is excited. Of course, because the system is open-loop, there are no means of knowing if the rotor has moved or not. To remove this uncertainty, it is conventional to use a rotor position detection scheme of some sort which provides a signal representative of rotor position. The excitation can then be applied as a function of the position. Such machines are often referred to as xe2x80x9crotor position switched machinesxe2x80x9d.
Since current in the windings is relatively easy to measure, closed-loop control is commonly accomplished by monitoring and controlling the energizing current in the windings. However, the desired output of the machine is usually torque, position or speed, and current has a highly non-linear relationship to all of these. The result is that current control techniques generally have inaccuracies in the output, such as torque ripple, position error or speed error.
A typical switched reluctance drive is shown in FIG. 2. In this example, the machine 36 corresponds to that shown in FIG. 1. The three phase windings A, B and C are switched onto a d.c. supply V by a set of power electronic switches 48. The moments at which the switches operate are determined by the controller 38, which may be implemented either in hardware or in the software of a microcontroller or digital signal processor. The firing signals are sent to the switches via a data bus 46. Closed loop current feedback is provided by sensing the phase currents by a current sensor 44 and feeding back a signal proportional to phase current. The control algorithms often include a proportional (P), proportional-plus-integral (P+I), time optimal, feedback linearized, proportional/integral/derivative (PID) function, or one of many others as is well understood in the art. It is also common for an outer control loop of position or speed to be provided by feeding back a rotor position signal from a position detector 40.
In operation, a current demand iD on line 42 is provided to the controller and this regulates the current in the windings, according to the particular control scheme adopted, to produce the desired output from the machine. Those skilled in the art will be familiar with the many variations of current controllers which exist, each of which has its own merits, but all of them suffer from the problems of non-linearity between the controlled variable and the machine output described above.
It has been recognized by the inventor that the more fundamental control variable in a switched reluctance machine is the flux which is set up in the magnetic circuit in the machine when a phase winding is energized. The flux is directly responsible for the force which acts on the rotor to urge it to a position of minimum reluctance, i.e. to pull the rotor round, with respect to the energized stator poles. Embodiments of this invention use closed loop determination and control of flux to achieve much better performance from the machine than has hitherto been possible with closed loop control of current.
In the paper xe2x80x98Torque Control of Switched Reluctance Drivesxe2x80x99 by P. G. Barrass and B. C. Mecrow, ICEM 96 Proceedings, International Conference on Electrical Machines, Sep. 10-12, 1996, Vigo, Spain, Vol 1, pp 254-259, incorporated herein by reference, there is a proposal to provide torque control by reference to flux linkage reference waveforms using a look-up table that stores fixed values of flux ramps for co-ordinates of supply voltage, phase current and rotor position. The flux values and co-ordinates are specific to a particular motor. At any instant the pre-stored values of flux and torque are chosen from fed back measurements of phase current and the stored machine data. There is a fixed relationship between the monitored variables and the values of the flux waveforms in the look-up table that are used to produce an output for a given motor. This system is essentially still a closed loop current controller, since the parameter fed back and the parameter controlled is current.
Up to now it has not been proposed to control flux without deriving or estimating values based on stored fixed values particular to a machine and its characteristics, based on the feedback of phase current.
According to embodiments of the present invention there is provided a brushless electrical machine comprising: a rotor; a stator; at least one phase winding arranged to establish flux in a magnetic circuit in the machine; and transducer means arranged in relation to the magnetic circuit to produce a flux signal indicative of the flux in a flux path associated with the at least one phase winding.
According to embodiments of the invention the machine, which can be run as a motor or a generator, derives the flux signal indicative of the flux itself from the magnetic circuit. The flux signal may be the output of a transducer arranged to measure directly the flux in the magnetic circuit.
The transducer means may be arranged directly in the flux path. To avoid the transducer being an excessive contributor to the reluctance of the magnetic circuit, it may be arranged in the flux path but so that it only takes up a fraction of the area of the flux path. The transducer means can conveniently be arranged in a recess of a pole face of a stator pole or be deposited on a pole face.
The transducer means can be any device known to produce an output that is indicative of the flux present. One example is a Hall-effect device that produces a voltage output that is directly proportional to the flux.
Embodiments of the invention enable direct flux control of the machine which has been found to be more accurate and is more amenable to on-line adaptation than the current-based control previously used. It uses a real-time determination of flux, as opposed to a selection from a set of stored values. It is, thus, adaptable to different types of machine and is not dedicated to a specific machine.
Embodiments of the invention also extend to a brushless electrical machine drive system comprising a brushless electrical machine having a rotor, a stator and at least one phase winding arranged to establish flux in a magnetic circuit in the machine; transducer means arranged in relation to the magnetic circuit to produce a flux signal indicative of the flux in a flux path associated with the at least one phase winding; switch means electrically connected with the at least one phase winding; and flux control means having an input signal representing the demanded output of the machine, which control means are responsive to the input signal and the flux signal to produce control signals for controlling the flux in the or each phase winding.
The flux control means are responsive to the input signal and the flux signal to produce the control signals according to a proportional, proportional-plus-integral, proportional/integral/derivative, time optimal or feedback linearized control law.
Preferably, the flux control means further includes means for timing the control signals for actuating the excitation means.
Embodiments of the invention also extend to a method of controlling a brushless electrical machine having a rotor, a stator and at least one phase winding, the method comprising: arranging transducer means in a magnetic circuit of the machine to produce a flux signal indicative of the flux in the at least one phase winding; producing an input signal representing the demanded output of the machine; controlling energization of the at least one phase winding in response to the input signal and the flux signal.